This invention relates to an automatic focusing system for a movie camera, and to a camera incorporating the same.
The parent application discloses an automatic focusing system for a movie camera wherein the lens mount is moved to a location, hereinafter called the focus location, at which an image of a subject is in focus on the focal plane of a recording station, in response to a range pulse generated on receipt of an echo from a subject illuminated with a supersonic burst transmitted from the camera. Arrival of the lens mount to its focus position keys the transducer and the cycle repeats allowing the lens mount to track a subject whose distance is changing. The tracking of a moving subject by the lens mount is consequently done in steps determined by the time required for the lens mount to move to its new focus position.
In the focusing system of the parent application, the range pulses define time intervals linearly related to subject distance by a fixed scale factor whereby the lens/subject function, which establishes the relationship between the distance of a subject and the position of the lens mount at which a subject at that distance is in focus, is parametrically related to time. The accumulated output of a scaled clock, whose pulse repetition frequency varies in accordance with the time derivative of an approximation of the lens/subject function, is the integral of the derivative of the function itself so that at the end of a time interval defined by a range pulse, the number of pulses produced by the scaled clock will be a number representing the focus position of the lens. This number representing the current focus position is stored in a register whose contents are subtracted from the contents of a register containing a number representing the previous focus position of the lens. The sign of the difference controls the direction of rotation of a reversible motor connected to the lens mount while the magnitude of the difference determines the distance the lens mount must move from the previous focus position to the new focus position. Thus, the absolute position of the lens mount (i.e., its position relative to a fixed position) is not known. Consequently, any error introduced in the actual positioning of the lens mount by a missed pulse, for example, during one ranging operation will remain during subsequent ranging operations.
It is therefore an object of the present invention to provide a new and improved automatic focusing movie camera having smooth and continuous lens movement.
Another object is to provide an automatic focusing system for a movie camera wherein errors that may be introduced during one ranging operation are eliminated by subsequent ranging operations.